Robert Ehrmann - Academia.edu (original) (raw)

Papers by Robert Ehrmann

Wind farm operators observe production deficits as machines age. Quantifying deterioration on ind... more Wind farm operators observe production deficits as machines age. Quantifying deterioration on individual components is difficult, but one potential explanation is accumulation of blade surface roughness. Historically, wind turbine airfoils were designed for lift to be insensitive to roughness by simulating roughness with trip strips. However, roughness was still shown to negatively affect performance. Furthermore, experiments illustrated distributed roughness is not properly simulated by trip strips. To understand how real-world roughness affects performance, field measurements of turbine-blade roughness were made and simulated on a NACA 63 3-418 airfoil in a wind tunnel. Insect roughness and paint chips were characterized and recreated as distributed roughness and a forward-facing step. Distributed roughness was tested in three heights and five density configurations. The model chord Reynolds number was varied between 0.8 to 4.8 × 10 6. Measurements of lift, drag, pitching moment, and boundary-layer transition were completed. Results indicate minimal effect from paint-chip roughness. As distributed roughness height and density increase, lift-curve slope, maximum lift, and lift-to-drag ratio decrease. As Reynolds number increases, bypass transition occurs earlier. The critical roughness Reynolds number varies between 178 to 318, within the historical range. Little sensitivity to pressure gradient is observed. At a chord Reynolds number of 3.2 × 10 6 , the maximum lift-to-drag ratio decreases 40% for 140 µm roughness, corresponding to a 2.3% loss in annual energy production. Simulated performance loss compares well to measured performance loss on an in-service wind turbine.

(NFAC) 40 foot by 80 foot wind tunnel at NASA Ames Research Center in the summer of 2011. The dev... more (NFAC) 40 foot by 80 foot wind tunnel at NASA Ames Research Center in the summer of 2011. The development of two measurement techniques is discussed in this work, both with the objective of making measurements on AMELIA for CFD validation. First, the work on the application of the Fringe-Imaging Skin Friction (FISF) technique to AMELIA is discussed. The FISF technique measures the skin friction magnitude and direction by applying oil droplets on a surface, exposing them to flow, measuring their thickness, and correlating their thickness to the local skin friction. The technique has the unique ability to obtain global skin friction measurements. A two foot, nickel plated, blended wing section test article has been manufactured specifically for FISF. The model is illuminated with mercury vapor lamps and imaged with a Canon 50D with a 546 nm bandpass filter. Various tests are applied to the wing in order to further characterize uncertainties related with the FISF technique. Human repeatability has uncertainties of ±2.3% of fringe spacing and ±2.0° in skin friction vector direction, while image post processing yields ±25% variation in skin friction coefficient. A method for measuring photogrammetry uncertainty is developed. The effect of filter variation and test repeatability was found to be negligible. A validation against a Preston tube was found to have 1.8% accuracy. Second, the validation of a micro flow measurement device is investigated. Anemometers have always had limited capability in making near wall measurements, driving the design of new devices capable of measurements with increased wall proximity. Utilizing a thermocouple boundary layer rake, wall measurements within 0.0025 inches of the surface have been made. A Cross Correlation Rake (CCR) has the advantage of not requiring calibration but obtaining the same proximity and resolution as the thermocouple boundary layer rake. The flow device utilizes time of flight measurements computed via cross correlation to calculate wall velocity profiles. The CCR was designed to be applied to AMELIA to measure flow velocities above a flap in a transonic flow regime. The validation of the CCR was unsuccessful. Due to the fragile construction of the CCR, only one data point at 0.10589 inches from the surface was available for validation. The subsonic wind tunnel's variable frequency drive generated noise which could not be filtered or shielded, requiring the use of a flow bench for validation testing. Since velocity measurements could not be made in the flow bench, a v comparison of a fast and slow velocity was made. The CCR was not able to detect the difference between the two flow velocities. Currently, the CCR cannot be applied on AMELIA due to the unsuccessfully validation of the device.

California Polytechnic Corporation, Georgia Tech Research Institute (GTRI), and DHC Engineering c... more California Polytechnic Corporation, Georgia Tech Research Institute (GTRI), and DHC Engineering collaborated on a NASA NRA to develop and validate predictive capabilities for the design and performance of Cruise Efficient, Short TakeOff and Landing (CESTOL) subsonic aircraft. In addition, a large scale wind tunnel effort to validate predictive capabilities for aerodynamic performance and noise during takeoff and landing has been undertaken. The model, Advanced Model for Extreme Lift and Improved Aeroacoustics (AMELIA), was designed as a 100 passenger, N+2 generation, regional, CESTOL airliner with hybrid blended wingbody with circulation control. The model design was focused on fuel savings and noise goals set out by the NASA N+2 definition. The AMELIA is 1/13 scale with a 10 ft wing span. PatersonLabs was chosen to build AMELIA and The National FullScale Aerodynamic Complex (NFAC) 40 ft by 80 ft wind tunnel was chosen to perform the nine week long large scale wind tunnel test in the summer of 2011.

32nd ASME Wind Energy Symposium, 2014

California Polytechnic Corporation, Georgia Tech Research Institute (GTRI), and DHC Engineering c... more California Polytechnic Corporation, Georgia Tech Research Institute (GTRI), and DHC Engineering collaborated on a NASA NRA to develop and validate predictive capabilities for the design and performance of Cruise Efficient, Short TakeOff and Landing (CESTOL) subsonic aircraft. In addition, a large scale wind tunnel effort to validate predictive capabilities for aerodynamic performance and noise during takeoff and landing has been undertaken. The model, Advanced Model for Extreme Lift and Improved Aeroacoustics (AMELIA), was designed as a 100 passenger, N+2 generation, regional, CESTOL airliner with hybrid blended wingbody with circulation control. The model design was focused on fuel savings and noise goals set out by the NASA N+2 definition. The AMELIA is 1/13 scale with a 10 ft wing span. PatersonLabs was chosen to build AMELIA and The National FullScale Aerodynamic Complex (NFAC) 40 ft by 80 ft wind tunnel was chosen to perform the nine week long large scale wind tunnel test in the summer of 2011.

40th Fluid Dynamics Conference and Exhibit, 2010

33rd Wind Energy Symposium, 2015

Over time it has been reported wind turbine power output can diminish below manufacturers promise... more Over time it has been reported wind turbine power output can diminish below manufacturers promised levels. This is clearly undesirable from an operator standpoint, and can also put pressure on turbine companies to make up the difference. A likely explanation for the discrepancy in power output is the contamination of the leading edge due to environmental conditions creating surfaces much coarser than intended. To examine the effects of airfoil leading edge roughness, a comprehensive study has been performed both experimentally and computationally on a NACA 633 − 418 airfoil. A description of the experimental setup and test matrix are provided, along with an outline of the computational roughness amplification model used to simulate rough configurations. The experimental investigation serves to provide insight into the changes in measurable airfoil properties such as lift, drag, and boundary layer transition location. The computational effort is aimed at using the experimental results to calibrate a roughness model that has been implemented in an unsteady RANS solver. Furthermore, a blade element momentum code was used to assess the impact on the performance of a turbine as whole due to discrepancies in clean vs. soiled airfoil characteristics. The results have implications in predicting the power loss due to leading edge surface roughness, and can help to establish an upper bound on admissible surface contamination levels.

51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013

28th AIAA Applied Aerodynamics Conference, 2010

51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013

A collaboration between California Polytechnic Corporation with Georgia Tech Research Institute (... more A collaboration between California Polytechnic Corporation with Georgia Tech Research Institute (GTRI) and DHC Engineering worked on a NASA NRA to develop predictive capabilities for the design and performance of Cruise Efficient, Short TakeOff and Landing (CESTOL) subsonic aircraft. The work presented in this paper gives details of a large scale wind tunnel effort to validate predictive capabilities for this NRA for aerodynamic and acoustic performance during takeoff and landing. The model, Advanced Model for Extreme Lift and Improved Aeroacoustics (AMELIA), was designed as a 100 passenger, N+2 generation, regional, cruise efficient short takeoff and land (CESTOL) airliner with hybrid blended wing-body with circulation control. AMELIA is a 1/11 scale with a corresponding 10 ft wing span. The National Full-Scale Aerodynamic Complex (NFAC) 40 ft by 80 ft wind tunnel was chosen to perform the large-scale wind tunnel test. The NFAC was chosen because both aerodynamic and acoustic measurements will be obtained simultaneously, the tunnel is large enough that the

31st AIAA Applied Aerodynamics Conference, 2013

52nd Aerospace Sciences Meeting, 2014

A surface roughness model extending the Langtry-Menter transition model has been implemented in a... more A surface roughness model extending the Langtry-Menter transition model has been implemented in a RANS framework. The model, originally proposed by Dassler, Kozulovic, and Fiala, introduces an additional scalar field roughness amplification quantity. This value is explicitly set at rough wall boundaries using surface roughness parameters and local flow quantities. This additional transport equation allows non-local effects of surface roughness to be accounted for downstream of rough sections. This roughness amplification variable is coupled with the Langtry-Menter model and used to modify the criteria for transition. Results from flat plate test cases show good agreement with experimental transition behavior on the flow over varying sand grain roughness heights. Additional validation studies were performed on a NACA 0012 airfoil with leading edge roughness. The computationally predicted boundary layer development demonstrates good agreement with the experimental results. New experimental tests using multiple roughness configurations were conducted to further validate and calibrate the model. Finally modifications are discussed to potentially improve the behavior of the Langtry-Menter transition model at high Reynolds numbers and angles of attack.

50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012

Wind farm operators observe production deficits as machines age. Quantifying deterioration on ind... more Wind farm operators observe production deficits as machines age. Quantifying deterioration on individual components is difficult, but one potential explanation is accumulation of blade surface roughness. Historically, wind turbine airfoils were designed for lift to be insensitive to roughness by simulating roughness with trip strips. However, roughness was still shown to negatively affect performance. Furthermore, experiments illustrated distributed roughness is not properly simulated by trip strips. To understand how real-world roughness affects performance, field measurements of turbine-blade roughness were made and simulated on a NACA 63 3-418 airfoil in a wind tunnel. Insect roughness and paint chips were characterized and recreated as distributed roughness and a forward-facing step. Distributed roughness was tested in three heights and five density configurations. The model chord Reynolds number was varied between 0.8 to 4.8 × 10 6. Measurements of lift, drag, pitching moment, and boundary-layer transition were completed. Results indicate minimal effect from paint-chip roughness. As distributed roughness height and density increase, lift-curve slope, maximum lift, and lift-to-drag ratio decrease. As Reynolds number increases, bypass transition occurs earlier. The critical roughness Reynolds number varies between 178 to 318, within the historical range. Little sensitivity to pressure gradient is observed. At a chord Reynolds number of 3.2 × 10 6 , the maximum lift-to-drag ratio decreases 40% for 140 µm roughness, corresponding to a 2.3% loss in annual energy production. Simulated performance loss compares well to measured performance loss on an in-service wind turbine.

(NFAC) 40 foot by 80 foot wind tunnel at NASA Ames Research Center in the summer of 2011. The dev... more (NFAC) 40 foot by 80 foot wind tunnel at NASA Ames Research Center in the summer of 2011. The development of two measurement techniques is discussed in this work, both with the objective of making measurements on AMELIA for CFD validation. First, the work on the application of the Fringe-Imaging Skin Friction (FISF) technique to AMELIA is discussed. The FISF technique measures the skin friction magnitude and direction by applying oil droplets on a surface, exposing them to flow, measuring their thickness, and correlating their thickness to the local skin friction. The technique has the unique ability to obtain global skin friction measurements. A two foot, nickel plated, blended wing section test article has been manufactured specifically for FISF. The model is illuminated with mercury vapor lamps and imaged with a Canon 50D with a 546 nm bandpass filter. Various tests are applied to the wing in order to further characterize uncertainties related with the FISF technique. Human repeatability has uncertainties of ±2.3% of fringe spacing and ±2.0° in skin friction vector direction, while image post processing yields ±25% variation in skin friction coefficient. A method for measuring photogrammetry uncertainty is developed. The effect of filter variation and test repeatability was found to be negligible. A validation against a Preston tube was found to have 1.8% accuracy. Second, the validation of a micro flow measurement device is investigated. Anemometers have always had limited capability in making near wall measurements, driving the design of new devices capable of measurements with increased wall proximity. Utilizing a thermocouple boundary layer rake, wall measurements within 0.0025 inches of the surface have been made. A Cross Correlation Rake (CCR) has the advantage of not requiring calibration but obtaining the same proximity and resolution as the thermocouple boundary layer rake. The flow device utilizes time of flight measurements computed via cross correlation to calculate wall velocity profiles. The CCR was designed to be applied to AMELIA to measure flow velocities above a flap in a transonic flow regime. The validation of the CCR was unsuccessful. Due to the fragile construction of the CCR, only one data point at 0.10589 inches from the surface was available for validation. The subsonic wind tunnel's variable frequency drive generated noise which could not be filtered or shielded, requiring the use of a flow bench for validation testing. Since velocity measurements could not be made in the flow bench, a v comparison of a fast and slow velocity was made. The CCR was not able to detect the difference between the two flow velocities. Currently, the CCR cannot be applied on AMELIA due to the unsuccessfully validation of the device.

California Polytechnic Corporation, Georgia Tech Research Institute (GTRI), and DHC Engineering c... more California Polytechnic Corporation, Georgia Tech Research Institute (GTRI), and DHC Engineering collaborated on a NASA NRA to develop and validate predictive capabilities for the design and performance of Cruise Efficient, Short TakeOff and Landing (CESTOL) subsonic aircraft. In addition, a large scale wind tunnel effort to validate predictive capabilities for aerodynamic performance and noise during takeoff and landing has been undertaken. The model, Advanced Model for Extreme Lift and Improved Aeroacoustics (AMELIA), was designed as a 100 passenger, N+2 generation, regional, CESTOL airliner with hybrid blended wingbody with circulation control. The model design was focused on fuel savings and noise goals set out by the NASA N+2 definition. The AMELIA is 1/13 scale with a 10 ft wing span. PatersonLabs was chosen to build AMELIA and The National FullScale Aerodynamic Complex (NFAC) 40 ft by 80 ft wind tunnel was chosen to perform the nine week long large scale wind tunnel test in the summer of 2011.

32nd ASME Wind Energy Symposium, 2014

California Polytechnic Corporation, Georgia Tech Research Institute (GTRI), and DHC Engineering c... more California Polytechnic Corporation, Georgia Tech Research Institute (GTRI), and DHC Engineering collaborated on a NASA NRA to develop and validate predictive capabilities for the design and performance of Cruise Efficient, Short TakeOff and Landing (CESTOL) subsonic aircraft. In addition, a large scale wind tunnel effort to validate predictive capabilities for aerodynamic performance and noise during takeoff and landing has been undertaken. The model, Advanced Model for Extreme Lift and Improved Aeroacoustics (AMELIA), was designed as a 100 passenger, N+2 generation, regional, CESTOL airliner with hybrid blended wingbody with circulation control. The model design was focused on fuel savings and noise goals set out by the NASA N+2 definition. The AMELIA is 1/13 scale with a 10 ft wing span. PatersonLabs was chosen to build AMELIA and The National FullScale Aerodynamic Complex (NFAC) 40 ft by 80 ft wind tunnel was chosen to perform the nine week long large scale wind tunnel test in the summer of 2011.

40th Fluid Dynamics Conference and Exhibit, 2010

33rd Wind Energy Symposium, 2015

Over time it has been reported wind turbine power output can diminish below manufacturers promise... more Over time it has been reported wind turbine power output can diminish below manufacturers promised levels. This is clearly undesirable from an operator standpoint, and can also put pressure on turbine companies to make up the difference. A likely explanation for the discrepancy in power output is the contamination of the leading edge due to environmental conditions creating surfaces much coarser than intended. To examine the effects of airfoil leading edge roughness, a comprehensive study has been performed both experimentally and computationally on a NACA 633 − 418 airfoil. A description of the experimental setup and test matrix are provided, along with an outline of the computational roughness amplification model used to simulate rough configurations. The experimental investigation serves to provide insight into the changes in measurable airfoil properties such as lift, drag, and boundary layer transition location. The computational effort is aimed at using the experimental results to calibrate a roughness model that has been implemented in an unsteady RANS solver. Furthermore, a blade element momentum code was used to assess the impact on the performance of a turbine as whole due to discrepancies in clean vs. soiled airfoil characteristics. The results have implications in predicting the power loss due to leading edge surface roughness, and can help to establish an upper bound on admissible surface contamination levels.

51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013

28th AIAA Applied Aerodynamics Conference, 2010

51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2013

A collaboration between California Polytechnic Corporation with Georgia Tech Research Institute (... more A collaboration between California Polytechnic Corporation with Georgia Tech Research Institute (GTRI) and DHC Engineering worked on a NASA NRA to develop predictive capabilities for the design and performance of Cruise Efficient, Short TakeOff and Landing (CESTOL) subsonic aircraft. The work presented in this paper gives details of a large scale wind tunnel effort to validate predictive capabilities for this NRA for aerodynamic and acoustic performance during takeoff and landing. The model, Advanced Model for Extreme Lift and Improved Aeroacoustics (AMELIA), was designed as a 100 passenger, N+2 generation, regional, cruise efficient short takeoff and land (CESTOL) airliner with hybrid blended wing-body with circulation control. AMELIA is a 1/11 scale with a corresponding 10 ft wing span. The National Full-Scale Aerodynamic Complex (NFAC) 40 ft by 80 ft wind tunnel was chosen to perform the large-scale wind tunnel test. The NFAC was chosen because both aerodynamic and acoustic measurements will be obtained simultaneously, the tunnel is large enough that the

31st AIAA Applied Aerodynamics Conference, 2013

52nd Aerospace Sciences Meeting, 2014

A surface roughness model extending the Langtry-Menter transition model has been implemented in a... more A surface roughness model extending the Langtry-Menter transition model has been implemented in a RANS framework. The model, originally proposed by Dassler, Kozulovic, and Fiala, introduces an additional scalar field roughness amplification quantity. This value is explicitly set at rough wall boundaries using surface roughness parameters and local flow quantities. This additional transport equation allows non-local effects of surface roughness to be accounted for downstream of rough sections. This roughness amplification variable is coupled with the Langtry-Menter model and used to modify the criteria for transition. Results from flat plate test cases show good agreement with experimental transition behavior on the flow over varying sand grain roughness heights. Additional validation studies were performed on a NACA 0012 airfoil with leading edge roughness. The computationally predicted boundary layer development demonstrates good agreement with the experimental results. New experimental tests using multiple roughness configurations were conducted to further validate and calibrate the model. Finally modifications are discussed to potentially improve the behavior of the Langtry-Menter transition model at high Reynolds numbers and angles of attack.

50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, 2012